Room-temperature structures of solid hydrogen at high pressures

Abstract

By employing first-principles metadynamics simulations, we explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially ordered hcp phase (po-hcp), a mixture of ordered graphene-like H 2 layers and the other layers of weakly coupled, disordered H 2 molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm -1 . At 275 GPa, our simulations predicted a transformation from po-hcp into the ordered molecular metallic Cmca phase (4 molecules/cell) that was previously proposed to be stable only above 400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic stabilities of the po-hcp and metallic Cmca phases over all known structures at 220-242 GPa and >242 GPa, respectively.Our simulations highlighted the major role played by temperature in tuning the phase stabilities and provided theoretical support for claimed metallization of solid hydrogen below 300 GPa at 300 K.2

Graphene physics and insulator-metal transition in compressed hydrogen

“…Though a graphene motif is most evident in the candidate structures Cc, Pc and Pbcn for phase IV [11][12][13][14] , it can be also found in other candidate structures proposed for dense hydrogen. Thus, the distorted hexagons are clearly seen in the layered C2/c and Cmca-4 structures.…”

“…The former is the most plausible candidate for the low temperature phase III and the latter is a predicted at higher pressures and low temperatures 7,11,12 . The intermolecular distance in graphene-like layeres of high-pressure structures like C2/c,Cc, Pc and Pbcn is on the order of 1.0-1.1Å [11][12][13][14] . This is noticeably shorter than the minimal distance separating two hydrogen atoms from the neighboring layers (1.4 -1.5Å).…”

“…A new phase (phase IV) has been discovered above 220 GPa in the higher temperature regime 4,5,8 . Though several different structures have been proposed for phase IV from first-principles calculations, 11,12,15 there is agreement that this phase can be viewed as a mixed-layer structure, where layers of weakly interacting H 2 molecules are sandwiched between graphene-like layers.…”

“…The notation of the symmetrical points is borrowed from Ref. 11 mixed phases like Cc, Pc and Pbcn [11][12][13][14] , where the layers of unbound H 2 molecules are sandwiched between the graphene-like layers. The situation with Pbcn structure, for example, is similar to that for C2/c.…”

“…Recently, solid hydrogen has been intensively investigated in a new pressure-temperature domain (200-360 GPa, and just above 300 K), both experimentally [4][5][6][7][8][9] and theoretically [10][11][12][13][14][15] . A new phase (phase IV) has been discovered above 220 GPa in the higher temperature regime 4,5,8 .…”